master e3b8ddd: Speed up by storing frame faces in hash tables instead of alists

[Eli Zaretskii]

Thanks for working on this important improvement.

However, this changeset removes the variable face-new-frame-defaults,
so code which references it will now signal an error.  (The variable
is declared obsolete, but that has no effect since the variable itself
doesn't exist.)  I don't think we can remove such a variable without
proper deprecation period.  So please add some code to compute that
variable at startup and whenever the faces are updated; we must keep
that variable and its supporting code until we actually remove the
variable.

Also, the C counterpart of face--new-frame-defaults should be changed
from Vface_new_frame_defaults to Vface__new_frame_defaults, and the
commentary before init_xfaces should be updated to that effect (it
currently still mentions the deleted variable
face-new-frame-defaults).